Conventional analog to digital convertors, such as those for digitising an analog input signal generated by a transducer (such as a strain gauge), generally include expensive high stability components arranged in complex circuits. The stability of the components is selected to minimize signal drift within the circuits, thereby providing an accurate digital representation of the analog input signal.
British Patent No. 2 077 537 A discloses a typical prior art analog to digital converter wherein a circuit for digitising an analog input signal is generated by a transducer such as a strain gauge. The circuitry comprises an analog to digital convertor connected to the output of the transducer by a semiconductor switch. The switch is controlled by a microprocessor which periodically changes over the switch so as to connect the analog to digital convertor input to a calibration signal derived from an energizing source. The microprocessor scales the output of the channel by a multiplying factor having a magnitude determined so that its product with the digitized calibration signal is always constant. Any drift at the input of the analog to digital convertor can therefore be compensated by changing the magnitude of the multiplying factor. This circuit, however, cannot continuously digitize an analog input signal because the input to the analog to digital convertor must be periodically disconnected from the input signal to compensate for input drift.